The present invention relates to analog-to-digital converters and, more particularly, to a high-speed analog-to-digital converter.
One type of analog-to-digital converter (A/D converter or ADC) is a single slope converter. Such a converter transforms input signals (voltage, current, charge, etc.) into a time interval having a duration proportional to the value of the input voltage. The time interval is measured by a digital counter in terms of an integral number of clock periods. The output of the counter represents the digitized input value. The speed of the A/D converter is limited by how quickly the number of bits in the converter may be resolved by the counter. For example, for a 10-bit 10 MHz converter, a 10-GHz counter is required. Such high-rate counters are not readily implemented in imbedded applications on integrated circuits such as modems, codecs, and single-chip cameras. However, the use of embedded high-speed A/D converters in such applications is highly desired. Single-slope A/D converters are especially attractive for embedded applications because they can potentially be designed to take up very little silicon area. Prior art systems have focused on improving performance by improving the speed of the counter, which has been unsatisfactory.
Time digitizers are circuits which measure time intervals in terms of an integral number of propagation delays or gate delays, or fractions thereof. PLL-based time digitizers have traditionally been used in high-speed instrumentation for digital IC testing and for particle physics experiments, but have not been used in relatively slower A/D converters.
Advantageously, it is recognized that such time digitizers can replace the limited counters in single slope A/D converters. An A/D converter combines a voltage-to-time-interval capability with a PLL-based time digitizer. The result is a high speed single-slope A/D capable of operating above 10 MHz. This converter dissipates very little power and consumes very little circuit area, making it especially suitable for embedded applications. Furthermore, the converter may be designed, with minimal additional circuitry, to be programmable so that conversion rate can be traded off with resolution. For example, the same converter having a 10-bit resolution with a conversion rate of 10 MHz can be electrically programmed to have an 8-bit resolution with a conversion rate of 40 MHz, a 6-bit resolution with a conversion rate of 160 MHz, as well as a 12-bit resolution with a conversion rate of 2.5 MHz or a 16-bit resolution with a conversion rate of 625 kHz.
The fabrication and cost considerations of using more components and complexity in incorporating time digitizers into A/D converters is far outweighed by the speed, resolution, and adjustable programmability of such A/D converters using time digitizers. The prior art relied on merely increasing the speed of inherently slow counters with limited performance characteristics, and the field of A/D conversion is considered to be distinct from the field of high speed instrumentation. Accordingly, in view of the limitations of the prior art, the advantages of implementing such time digitizers from high speed instrumentation into A/D converters are both numerous and significant in improving the performance of A/D converters.